1. Field of the Invention
This invention relates to a lead frame which provides lead wires for an electronic part such as an intergrated circuit (IC) and which serves as a frame in the process of manufacturing the electronic part.
2. Description of the Prior Art
In assembling semiconductor devices such as transistors and integrated circuits, the semiconductor devices are often assembled successively on a lead frame having a metal ribbon with a plurality of metal wires formed by blanking. The lead frame includes portions on which semiconductor chips are fixedly set, portions used for wire bonding, and portions which are used as external lead wires when the semiconductor devices have been assembled.
FIG. 1 is a plane view of a conventional lead frame showing only a part of the lead frame which supports one semiconductor device. The lead frame is for a resin mold type semiconductor device. The lead frame has a square chip fixing plate 2 on which a semiconductor chip 1 is fixedly placed. A rectangular connecting frame plate 3 has one side which extends perpendicularly from the centers of two opposed sides of the chip fixing plate 2. A plurality of external connection electrode plates 4 are also provided and each electrode plate 4 has a lead wire connecting portion 4a which is used for wire bonding and a lead wire portion 4b. The lead wire connecting portions 4a confront the chip fixing plate 2 and the lead wire portion 4b is connected to the connecting frame plate 3.
When a semiconductor device is fixed to the lead frame described above, the semiconductor chip 1 is fixedly set on the chip fixing plate 2 and bonding pads 1a on the semiconductor chip 1 are connected to the lead wire connecting portions 4a with thin conductors 5 by bonding. Thereafter, the entire region indicated by the two-dot chain line in FIG. 1 identified by reference number 19 is molded with resin and then the lead frame is cut along the two-dot chain lines in FIG. 1 identified by reference number 20 to remove the part of the connecting frame plate 3 which becomes unnecessary after the semiconductor device has been molded in place. This forms a resin mold type semiconductor device.
When the semiconductor chip 1 is fixiedly set on the chip fixing plate 2 of the lead frame, both the chip 1 and plate 2 are flat as shown in FIG. 2(A). However, since the thermal coefficient of expansion of the chip 1 is different from that of the plate 2, when the temperature is raised in the resin molding operation, the semiconductor chip 1 and the chip fixing plate 2 become deformed as shown in FIG. 2(B). As a result of this deformation, stresses are set up in the semiconductor chip 1 and the chip fixing plate 2. If a large semiconductor chip is to be fixed to the lead plate, a rather large area is used for fixing the semiconductor chip 1 to the chip fixing plate 2 which produces rather large temperature induced stresses. As the size of the semiconductor chip increases, so do the stresses. Accordingly, when the semiconductor chip 1 is 1.times.1 cm.sup.2, it is sometimes cracked because the termperature induced stresses exceed a value allowable for the chip 1.